Rot happens, even to the best of houses. All it takes is wood, water, and warmth, and before you know it solid lumber turns to mush. Exterior trim is the most vulnerable to attack by rot fungi, and it doesn't have to be very old; the trim shown here was installed only 10 years ago.
Fortunately, rotted trim is generally easy to repair. (Rot-infested framing or mudsills pose a much bigger problem.) But before you can fix it, you have to find it. With screwdriver or awl in hand, scrutinize areas that are nearly horizontal and don't drain well, such as windowsills, drip caps, and water tables. Look for paint that is cracked, peeling, or blistering, or wood that's darker than the surrounding area or green with algae. Probe anywhere there's end grain, which wicks up water like a celery stalk in a grade-school science experiment. Pay particular attention to joints, which dry slowly, and to all wood that's close to dirt, concrete, or masonry. If you're able to push the tool's tip easily into a suspect board, then it's time to root out the rot.
Remove the Cap
After slicing through the old caulk around the plinth (to avoid damaging the adjacent trim), Vietri pries off the cap molding and levers the plinth free. Its water-blackened backside shows how extensively moisture penetrated this assembly.
Vietri protects the framing with overlapping strips of sheet-lead flashing. (Copper or waterproofing membrane also work.) Then he protects all the exposed edges of the old trim with a coat of oil-based primer.
Apply Adhesive to Trim Edges
When the primer dries, Vietri squeezes a bead of polyurethane construction adhesive over the edges of the old wood trim. He immediately beds the new plinth's mitered side pieces into the goop, which is both strong and waterproof.
Install New Cap
A PVC-solvent-based cement bonds the mitered pieces of the new plinth to each other and makes the joints waterproof. Vietri also fastens the pieces together with stainless steel trim-head screws. Dabs of acrylic glazing putty hide the screw heads.
Prep for Paint
A light sanding with 220-grit paper readies the plinth for a coat of acrylic-based primer and two coats of acrylic paint. The paint blends the repair with the rest of the trim, but it isn't needed to protect the PVC from the sun.